The present invention relates to a fluid rotating apparatus such as a vacuum pump or a compressor.
FIG. 14 shows a conventional vacuum pump provided with only one rotor. In the single rotor type vacuum pump, upon rotation of a rotor 101, two blades 102 and 102 inserted into the rotor 101 in the diameter direction of the rotor 101 rotate within a fixed cylindrical wall (stator), with the leading ends thereof in contact with the fixed wall because the two blades are urged in the diameter direction of the rotor by the operation of a spring 104. As a result, the capacity of each of two spaces 105, 105, partitioned by the two blades, within the fixed wall changes. Consequently, suction compression of the fluid occurs, and fluid which has flowed in through a suction port 106 provided in the fixed wall flows out through a discharge port 107 provided with a discharge valve. In the vacuum pump, in order to prevent internal leakage, it is necessary to seal the side surfaces and the leading ends of the blades 102, the side surfaces of the fixed wall 103, and the side surfaces of the rotor 101 with an oil film, respectively. Consequently, when this vacuum pump is used in a semiconductor manufacturing process such as CVD or dry etching in which a very corrosive reactive gas such as chlorine gas is used, the gas reacts with the sealing oil to form a reaction product in the pump. In such cases, it is necessary to perform maintenance operations frequently to remove the reaction product. In each maintenance operation, it is necessary to clean the pump to eliminate the reaction product and replace the oil. To do this, it is necessary to stop the operation of the process, and hence the operation rate is reduced. So long as the sealing oil is used in the vacuum pump, oil diffuses from the downstream side to the upstream side, thus polluting the vacuum chamber and deteriorating the performance of the process.
In view of these issues, a screw type vacuum pump has been developed and put into practical use as a dry pump which eliminates the use of sealing oil. FIG. 15 shows an example of a screw type vacuum pump. Two rotors 112 and 112, the rotational shafts of which are parallel with each other, are provided in a housing 111, and spiral grooves are formed on the peripheral surfaces of the rotors 112 and 112 between which a closed space is formed by engaging a concave portion (groove) 113a of one rotor with a projection 113b of the other rotor. Upon rotation of both rotors 112 and 112, the capacity of the closed space changes, and thus a suction/discharge operation is performed.
In the conventional two rotor type vacuum pump the two rotors 112 and 112 are synchronously rotated by a timing gear. That is, the rotation of a motor 115 is transmitted from a driving gear 116a to an intermediate gear 116b and then, to one of timing gears 116c and 116c in mesh provided on the shaft of each rotor 112. The phase of the rotational angles of both rotors 112 and 112 are adjusted by the engagement between the two timing gears 116c and 116c. Since gears are used for the power transmission of the motor and the synchronous rotation of the rotors in this type of vacuum pump, the vacuum pump is constructed to supply lubricating oil provided in a machine operation chamber 117 accommodating the above-described gears to the above-described gears. In addition, a mechanical seal 119 is provided between a fluid operating chamber 118 and the machine operation chamber 117 so that the lubricating oil does not penetrate into the fluid operating chamber 118 accommodating the rotors.
The two-rotor type vacuum pump of the above-described construction is beset with the following problems. (1) it is necessary to periodically replace the seals due to abrasion of the mechanical seal, and thus the pump is not maintenance-free; (2) mechanical loss is great because sliding torque is great due to the mechanical seal; (3) many gears are required for the power transmission and the synchronous rotation, i.e. many parts are required, so that the apparatus has a complicated construction; (4) synchronous rotation is obtained by use of contact type gears, so that high speed rotation is impossible and the apparatus is large-sized.